1. Field of the Invention
The present invention relates to an imaging device and more particularly to an autofocus technique (AF technique).
2. Description of the Related Art
In a phase difference AF (autofocus) technique, the focus lens position is detected based on the following principle. More specifically, “two luminous fluxes” (partial luminous fluxes) from the shooting optical system are collected by respective image reforming lenses (separator lenses) or other lenses and received by respective photoelectric element arrays (also referred to as the distance measurement sensors). The two luminous fluxes originate from the same point on the subject and have passed through regions of the shooting lens different from each other (e.g., regions symmetrical with respect to the optical axis). The amount and direction of displacement from the position of the focus lens (also referred to as the in-focus position) adapted to bring the subject into focus are detected by detecting the relative positional relationship between the subject images received by the two distance measurement sensors (distance measurement sensor pair). It should be noted that the distance measurement sensor pair obtains the change in gray level of each of the subject images in a predetermined direction (direction in which the photoreceiving element array is arranged in each distance measurement sensor) and performs distance measurement based on the change in gray level of each of the subject images (divided images) in the predetermined direction. That is, the distance measurement sensor pair provides distance measurement sensitive to the gray level change in the predetermined direction of the subject images.
In some phase difference-based focal point detectors, on the other hand, a plurality of pairs of distance measurement sensors (also referred to as the distance measurement sensor pairs) as described above are provided so as to detect the focus lens position relating to the subject at a plurality of positions (distance measurement points) in a shot image.
Examples of such a technique are described in Japanese Patent Laid-Open No. 2003-107323 (referred to as Patent Document 1) and Japanese Patent Laid-Open No. 2007-139935 (referred to as Patent Document 2).
The focal point detectors described in Patent Documents 1 and 2 have a condenser lens, stop mask, separator lens and distance measurement sensors. A total of three condenser lenses are provided as a condenser lens. More specifically, one of the condenser lenses (also referred to as the center lens) is provided at the center of the shooting lens through which the optical axis passes. Two other lenses (also referred to as the side lenses) are arranged one on each side of the condenser lens at the center.
As for the distance measurement points provided near the center of the shot image, distance measurement (focal point detection) is performed using the light passing through the “center lens” of the three condenser lenses. As for the distance measurement points provided relatively far from the center of the shot image, on the other hand, distance measurement is performed using the light passing through the “side lenses.”